1. Field of the Invention
This invention generally relates to composites of thermosetting resins and carbon fibers having polyhydroxyether sizings located at the interface (interphase region) of the fiber surfaces and the resin.
2. Description of the Prior Art
Polymeric matrix composites, also known as "fiber-reinforced plastics", involve products in which a polymeric matrix is combined with reinforcing fibers. The reinforcing fiber is embedded in the polymeric matrix material, and the polymeric matrix protects the brittle reinforcing fiber and transfers the load to the fiber. Composites advantageously combine strength, light weight, versatility in shape selection, and corrosion resistance, among other things.
Composites formed of glass fiber-reinforced thermosetting resins have been widely used for a number of years. However, in the case of such prior glass fiber-reinforced composites, a problem stems from the poor moisture stability of the glass fibers. That is to say, the glass fibers sustain structural damage when moisture (liquid or vapor infiltrating the resin matrix) contacts the fiber surfaces and the moisture is alternately frozen and thawed. This scenario arises when the glass fiber-reinforced composite part is subjected to regular outdoor exposures in a moist climate including temperature swings or temperature cycling over the freezing temperature of water. As a consequence, the glass fibers become fatigued, embrittled and ultimately fail. Therefore, despite the relative low cost of glass fiber-reinforced composites, they remain ill-suited for many outdoor applications available for composites.
Carbon fibers have been explored as an alternative type of high strength, fibrous reinforcement for resin matrices of composites. Carbon fibers have better environmental resistance against moisture, lighter weight (lower density), and higher stiffness as compared to glass fibers. However, the problem encountered with usage of carbon fibers in prior composites has been poor product durability. This poor durability of the composite has been attributed to bonding failure occurring at the interphase region of the resin matrix and the carbon fiber surfaces. As a consequence, the resin matrix eventually jiggles loose from the carbon fibers, whether in discrete fiber form or in fabric form, to cause failure of the composite.
The interphase region is a region of finite mass located at the fiber/matrix interface. This region has gradients in physical properties that greatly influence the performance of the final composite.
Carbon fibers and vinyl ester resins, in particular, have a compatibility problem which has proved a considerable prior obstacle to the use of these systems. This incompatibility has frustrated prior efforts to achieve an appreciable bond between this matrix system and carbon fiber surfaces per se. This has been disappointing because considerable cost advantages can be garnered from the carbon fiber-reinforced vinyl ester system due to the initial low material cost of the vinyl ester polymer, and the short time and low temperature required to fully cure these systems. The short hardening times and lower cure temperatures associated with this resin system translate into reduced production cycles which further reduces production costs.
In light of such fiber and resin compatibility problems, the character of the bond between fiber and matrix in composite materials has attracted considerable prior interest and scrutiny in the field, as indicated by the following patents.
Polyhydroxyethers are known interface materials, i.e., sizing materials, for glass fiber reinforced composites. The process of applying such sizing materials to a fiber surface prior to its impregnation with the matrix material is generally referred to as sizing. As to carbon fiber reinforced composites, U.S. Pat. No. 4,781,947 to Saito et al. teaches a sizing agent applied as a precoating to a carbon fiber, where the sizing agent is an unsaturated urethane compound produced by a reaction of an unsaturated alcohol with an isocyanate which is able to couple the carbon fiber with an unsaturated polyester resin or vinylester resin. The function of the sizing agent is to hydrogen bond to the fiber via the urethane bond and to crosslink with the matrix via the double bond present in the sizing material.